Nutating pumps are pumps having a piston that both rotates about its axis and contemporaneously slides axially and reciprocally within a liner or casing. With a full pump chamber, as the piston is rotated 360° about its axis, the piston slides axially through a dispense stroke and returns to its initial position after an intake or “fill” stroke. The combined 360° rotation and reciprocating axial movement of the piston produces a sinusoidal dispense profile illustrated in FIG. 1. The line 1 graphically illustrates the flow rate at varying points during one revolution of the piston. The portion of the curve 1 above the horizontal line 2 representing a zero flow rate represents the dispense or output stroke while the portion of the curve 1 disposed below the line 2 represents the intake or fill stroke.
Further, because the output is not linear (see the line 1 of FIG. 1), some users limit the operation of conventional nutating pumps to complete 360° revolutions of the piston or at least one full dispense stroke. However, this methodology often requires a user to choose between a small pump that requires multiple revolutions of the piston to dispense the required volume and a large pump that requires a partial revolution of the piston to dispense the required volume. Further, the operator may also have to choose between running the motor of a small pump at high speeds to dispense larger volumes and running the motor of a large pump at slow or minimum speeds for smaller volumes.
To avoid this dilemma, stepper motors have been used with nutating pumps to provide a partial revolution dispense. While using a partial revolution to accurately dispense fluid from a nutating pump is difficult due to the non-linear output of the nutating pump dispense profile (i.e., see FIG. 1), controllers, software algorithms and sensors can be used to monitor the angular position of the piston. Using this angular position, the controller can calculate the number of steps required to achieve the desired output as disclosed in U.S. Pat. No. 6,749,402, which is incorporated herewith. The sinusoidal profile illustrated in FIG. 1 is based upon a nutating pump operating at a constant motor speed. While operating the nutating pump at a constant motor speed has its benefits in terms of simplicity of controller design and pump operation, the use of a constant motor speed has inherent disadvantages.
Specifically, in certain applications, the maximum output flow rate illustrated on the left side of FIG. 1 can be disadvantageous because the output fluid may splash or splatter as the fluid is pumped into the output receptacle at the higher flow rates. For example, in paint or cosmetics dispensing applications, any splashing of the colorant as it is being pumped into the output container results in an inaccurate dispense as well as colorant being splashed on the machine, which requires labor-intensive clean up and maintenance. This splashing problem will adversely affect any nutating pump application where precise amounts of output fluid are being delivered to small receptacles or to output receptacles that are either full or partially full of liquid.
For example, the operation of a conventional nutating pump having the profile of FIG. 1 results in pulsed output flow as shown in FIGS. 2 and 3. The pulsed flow shown at the left in FIGS. 2 and 3, at speeds of 800 and 600 rpm respectively, results in pulsations 3 and 4, which are a cause of unwanted splashing. FIGS. 2 and 3 are renderings of actual digital photographs of an actual nutating pump in operation. While reducing the motor speed from 800 to 600 rpm results in a smaller pulse 4, the reduction in pulse size is minimal and the benefits are offset by the slower operation. To avoid splashing altogether, the motor speed would have to be reduced more than 20% thereby making the choice of a nutating pump less attractive despite its high accuracy.
A further disadvantage to the sinusoidal profile of FIG. 1 is an accompanying pressure spike that causes an increase in motor torque. Specifically, the large pressure drop that occurs within a nutating pump as the piston rotates from the point where the dispense rate is at a maximum to the point where the intake rate is at a maximum (i.e. the peak of the curve shown at the left in FIG. 1 to the valley of the curve shown towards the right in FIG. 1) can result in motor stalling for those systems where the motor is operated at a constant speed. Motor stalling will result in an inconsistent or non-constant motor speed, thereby affecting the sinusoidal dispense rate profile illustrated in FIG. 1 and any control system or control method based upon a preprogrammed sinusoidal dispense profile. The stalling problem will occur on the intake side of FIG. 1 as well as when the pump goes from the maximum intake flow rate to the maximum dispense flow rate.
The splashing and stalling problems are addressed in U.S. Pat. No. 6,749,402, specifically in FIG. 4, which shows a modified dispense profile 1a where the motor speed is varied during the pump cycle to flatten the curve 1 of FIG. 1. The variance in motor speed results in a reduction of the peak output flow rate while maintaining a suitable average flow rate by (i) increasing the flow rates at the beginning and the end of the dispense portion of the cycle, (ii) reducing the peak dispense flow rate, (iii) increasing the duration of the dispense portion of the cycle and (iv) reducing the duration of the intake or fill portion of the cycle. This is accomplished using a computer algorithm that controls the speed of the motor during the cycle thereby increasing or decreasing the motor speed as necessary to achieve a dispense curve like that shown in FIG. 4.
However, the nutating pump design of U.S. Pat. No. 6,749,402 as shown in FIG. 4, while reducing splashing, still results in a start/stop dispense profile and therefore the dispense is not a pulsation-free or a steady, smooth flow. Despite the decrease in peak dispense rate, the abrupt increase in dispense rate shown at the left of FIG. 4 and the abrupt drop off in flow rate shown near the center of FIG. 4 still provides for the possibility of some splashing. Further, the abrupt starting and stopping of the dispensing followed by a significant lag time during the fill portion of the cycle still presents the problems of significant pressure spikes and gaps in the fluid stream exiting the dispense nozzle. Any decrease in the slope of the portions of the curves shown at 1a, 1c would require an increase in the cycle time as would any decrease in the maximum fill rate. Thus, the only modifications that can be made to the cycle shown in FIG. 4 to reduce the abruptness of the start and finish of the dispensing portion of the cycle would result in increasing the cycle time and/or reducing the maximum fill rate.
Turning to FIG. 5, the dual-chamber nutating pump 20 of U.S. Pat. No. 7,946,832 is shown. The dual chamber pump 20 includes a rotating and reciprocating piston 10 that is disposed within a pump housing 21. The pump housing 21 is coupled to an enclosure 22 as well as to an intermediate housing 23 used primarily to house the coupling 24 that connects the piston 10 to the drive shaft 25, which in turn, is coupled to the motor 26. The coupling 24 is connected to the proximal end 30 of the piston 10 by a link 27 (see FIG. 6). A proximal section 28 of the piston 10 has a first maximum outer diameter that is substantially less than a second maximum outer diameter of the larger pump section 29 of the piston 10. The purpose of the larger maximum outer diameter of the pump section 29 of the piston 10 is the creation of a second pump chamber 44 in addition to the first pump chamber 42. The proximal section 28 connects to the pump section 29 at a beveled transition section 31. The pump section 29 of the piston 10 passes through a middle seal 32. The distal end 33 of the pump section 29 of the piston 10 is received in a distal seal 34. The fluid inlet is shown at 35 and the fluid outlet is shown at 36. The proximal section 28 of the piston 10 passes through a proximal seal 38 disposed within the seal housing 39.
The first pump chamber 42 is an area where fluid is primarily displaced by the axial movement of the piston 10 towards the end cap 22 as well as the rotation of the piston 10 and the engagement of fluid disposed in the first chamber 42 by the machined flat area 13. A conduit or passage 43 connects the first chamber 42 to the second chamber 44. The beveled transition section 31 between the outer diameters of the proximal section 28 and the larger pump section 29 of the piston 10 generates displacement through the second chamber 44.
The piston 10 is shown at the middle of its stroke in FIG. 5 as the end 33 of the pump section 29 of the piston 10 approaches the head 22. Fluid is forced out of the first chamber 42 and into the passage 43 (see the arrow 46). This action displaces fluid disposed in the passage 43 and causes it to flow around the proximal section 28 and transition section 31 of the piston 10, or through the second chamber 44 as shown in FIG. 5. It will also be noted that the flat or machined area 13 of the piston 10 has been rotated thereby also causing fluid flow in the direction of the arrow 46 through the passage 43 and towards the second chamber 44. FIG. 6 illustrates a reciprocating movement back towards the top of the intake stroke. The piston 10 moves in the direction of the arrow 47, which causes the transition section 31 to enter the second chamber 44 thereby causing fluid to be displaced through the outlet 36 or in the direction of the arrow 48. No fluid is being pumped from the first chamber 42 in FIG. 6 but, instead, the first chamber 42 is being loaded with fluid entering through the inlet 35 and flowing into the chamber 42 in the direction of the arrow 49.
Instead of all of the fluid in the first chamber 42 being dispensed during the first 180° of rotation of the piston 10 as with conventional nutating pumps (see FIG. 1), a portion of the fluid pumped from the first chamber 42 is pumped from the second chamber 44 during second 180° of rotation of the piston 10, or during the fill portion of the of the cycle illustrated in FIG. 6. In other words, a portion of the fluid being pumped is temporarily stored in the second chamber 44 and the stored fluid is then dispensed during the fill portion of the cycle as opposed to all of the fluid being dispensed during the dispense portion of the cycle as illustrated in FIG. 1. As a result, the output flow during the first 180° of rotation of the piston 10 is reduced and some of that flow is pumped out of the second chamber 44 during the subsequent second 180° of rotation of the piston 10 during the fill portion of the cycle.
Turning to FIG. 7, a dispense profile is shown for a dual-chamber pump 20 constructed in accordance with FIGS. 5-6 and operating at a constant motor speed of 800 rpm. Two dispense portions are shown at 1d and 1e and a fill portion of the profile is shown at 1f. A break in dispensing occurs at the beginning of the fill portion of the cycle and moderated dispense flows are shown by the curves 1d, 1e. 
However, the dual-chamber pump 20 of FIGS. 5-7, despite the improvements, can create pulsations, which can lead to splashing and inaccurate dispenses. Further, as shown by the non-linear dispense profile of FIG. 7, the pump 20 would need to be equipped with a sophisticated control system and feedback control components in order to accurately dispense a volume of fluid less than the volume dispensed during a full cycle. Accordingly, there is a need for an improved nutating pump, also adapted for mixing and having multiple pump chambers, with improved control and/or a method of control thereof whereby the pump motor is controlled so as to reduce the likelihood of splashing and pulsing during a dispense without compromising pump speed and accuracy.